Conventionally, a continuously variable transmission including a belt-type continuously variable transmission mechanism that includes a belt wound around a primary pulley and a secondary pulley has widely been adopted as an automatic transmission. This continuously variable transmission is provided with a forward clutch and a reverse brake (hereinafter, these will also simply be referred to as clutches) that are interposed between the continuously variable transmission mechanism and an engine, and engagement and disengagement (release) of the clutches are controlled in accordance with an operation of a selection lever (also called a shift lever) by a driver.
For example, in a case where the driver switches the selection lever from a P range or an N range (hereinafter referred to as a non-travel range) to a D range or an R range (hereinafter referred to as a travel range), a motion of the selection lever is transmitted to a manual valve by a physical interlocking mechanism or a physical and electrical interlocking mechanism. The manual valve is displaced to a position at which the manual valve communicates between a clutch source pressure and a piston oil chamber of the clutch, and the clutch is engaged when a hydraulic pressure is supplied to a piston oil chamber. In this way, an engine torque is transmitted to the continuously variable transmission.
An operation position of the selection lever is detected by an inhibitor switch, and an electrical signal that corresponds to the operation position is inputted to a controller. In a case where a switching of the selection lever from the non-travel range to the travel range is detected by the inhibitor switch, a control for gradually increasing a clutch instruction (command) pressure (hereinafter referred to as a selection control) is ordinarily carried out to engage the clutch.
However, there is a case where a deviation occurs between the output signal from the inhibitor switch and a communication state of the manual valve, which is caused by an assembling error, rattling, or the like of the interlocking mechanism between the selection lever and the manual valve. That is to say, there is a case where, despite a fact that a signal indicating that “the selection lever has been switched from the non-travel range to the travel range” is inputted from the inhibitor switch, the manual valve is brought into a state of being not completely displaced to a travel range position (hereinafter, such a state as described above is referred to as a “pseudo D state”).
In a case where the driver assumes that the switching of the selection lever to the travel range has been completed and depresses an accelerator in such a pseudo D state as described above, the clutch is actually not engaged, and thus an engine speed is rapidly increased by an operation of the accelerator. In a case where the manual valve is displaced to the travel range position in this state and is brought into the communication state with an oil passage for achieving the travel range, the clutch is engaged, and a torque is instantaneously inputted to the belt of the continuously variable transmission. In this case, a pulley oil (hydraulic) pressure (primary pressure and secondary pressure) for transmitting a torque becomes insufficient and there is a possibility that a belt slippage occurs.
A technique for suppressing the belt slippage by lowering the torque that is inputted to the pulley in a case where the pseudo D state is generated and the engine speed is rapidly increased as described above has been proposed. For example, a patent document 1 discloses a configuration of lowering the clutch instruction (command) pressure to be lower than that during the ordinary selection control and of limiting the engine output in a case where a racing of the engine occurs in the pseudo D state. With such a configuration, the engine output is promptly restricted during a non-engagement of the clutch, so as to be able to suppress the racing of the engine, an engagement shock of the clutch, and the belt slippage.
By the way, it is considered that, in a case where the driver slowly operates the selection lever from the non-travel range to the travel range, the pseudo D state, in which the deviation occurs between the output signal from the inhibitor switch and the communication state of the manual valve, is likely to be generated and, furthermore, this state continues. At this time, the engine speed is rapidly increased if the driver depresses an accelerator pedal. Thus, the belt slippage and the like can be suppressed by using the technique of the above-described patent document 1.
However, in a case where the selection lever is slowly operated and a time difference is present between a time at which the pseudo D state is generated and a time at which the pseudo D state is determined, the clutch instruction (command) pressure is, ordinarily, gradually increased according to the selection control. In this state, when the manual valve brings the oil passage for achieving the travel range into the communication state, an actual clutch pressure is rapidly increased to the clutch instruction (command) pressure and, thus, the clutch is rapidly engaged. Accordingly, the torque is instantaneously inputted to the belt of the continuously variable transmission and there is a possibility that the belt slippage occurs due to a shortage of the pulley pressure.
To handle this, it is considered to maintain the pulley pressure in a high state previously, in order to suppress the instantaneous torque inputted to the belt of the continuously variable transmission and occurrence of the belt slippage. However, this leads to an increase in a load on an oil pump for supplying the hydraulic pressure. In addition, in a case where this oil pump is driven by the engine, this also leads to a degradation of a fuel economy (consumption). Thus, a reduction in the load on the oil pump is desired.